1. Field of the Invention
The present invention relates to a glass substrate employed as a substrate in magnetic recording media such as hard disks, and to a magnetic recording medium comprising the above substrate.
With the development of information-related infrastructure such as the Internet, the need for information recording media such as magnetic disks and optical disks has increased sharply. The main structural components of the magnetic memory devices of computers and the like are magnetic recording media and magnetic heads for magnetic recording and reproduction. Known magnetic recording media include flexible disks and hard disks. Of these, examples of the substrate materials employed in hard disks (magnetic disks) include aluminum substrates, glass substrates, ceramic substrates, and carbon substrates. In practical terms, depending on size and application, aluminum substrates and glass substrates are primarily employed. In the hard disk drives of laptop computers, in addition to impact resistance, higher density recording, and the development of thinner magnetic recording media, the requirements of increased surface smoothness of the disk substrate and the development of thinner substrates are intensifying. Thus, there are limits to how well aluminum substrates, with afford poor surface hardness and rigidity, can respond. Accordingly, the development of glass substrates is currently the. mainstream (for example, see Document 1 (Published Japanese Translation of a PCT international publication for patent application (TOKUHYO) No. Heisei 9-507206), Document 2 (Japanese Unexamined Patent Publication (KOKAI) No. 2007-51064), Document 3 (Japanese Unexamined Patent Publication (KOKAI) No. 2001-294441), Document 4 (Japanese Unexamined Patent Publication (KOKAI) No. 2001-134925), Document 5 (Japanese Unexamined Patent Publication (KOKAI) No. 2001-348246), Document 6 (Japanese Unexamined Patent Publication (KOKAI) No. 2001-58843), Document 7 (Japanese Unexamined Patent Publication (KOKAI) No. 2006-327935), Document 8 (Japanese Unexamined Patent Publication (KOKAI) No. 2005-272212), or English language family members US 2005/215414A1 and U.S. Pat. No. 7,687,419, Document 9 (Japanese Unexamined Patent Publication (KOKAI) No. 2004-43295), Document 10 (Japanese Unexamined Patent Publication (KOKAI) No. 2005-314159), or English language family members US 2005/244656A1 and U.S. Pat. No. 7,595,273; which are expressly incorporated herein by reference in their entirety).
In recent years, with the goal of achieving even higher density recording in magnetic recording media, the use of magnetic materials of high magneto-anisotropic energy (magnetic materials of high Ku value), such as Fe-Pt and Co-Pt based materials, is being examined (for example, see Document 11 (Japanese Unexamined Patent Publication (KOKAI) No. 2004-362746) or English language family members US 2004/229006A1 and U.S. Pat. No. 7,189,438; which is expressly incorporated herein by reference in its entirety). It is necessary to reduce the particle diameter of the magnetic particles to achieve higher density recording. However, when just the particle diameter is reduced, the deterioration of magnetic characteristics due to thermal fluctuation becomes a problem. Magnetic materials of high Ku value tend not to be affected by thermal fluctuation, and are thus expected to contribute to the achievement of greater recording density.
However, the above-described magnetic materials of high Ku value must be in a specific state of crystal orientation to exhibit a high Ku value. Thus, a film must be formed at high temperature or thermoprocessing must be conducted at high temperature following film formation. Accordingly, the formation of a magnetic recording layer comprised of such magnetic materials of high Ku value requires that a glass substrate have high heat resistance that is capable of withstanding the above-described processing at high temperatures, that is, have a high glass transition temperature.
By the way, in disk-shaped magnetic recording media, data are written and read in the direction of rotation by radially displacing a magnetic head while rotating the medium at high speed about a center axis. In recent years, the rotational speed has been increased from 5,400 rpm to 7,200 rpm, and up to a high speed of 10,000 rpm to increase the writing rate and reading rate. However, in disk-shaped magnetic recording media, since the positions at which data are recorded are assigned in advance based on the distance from the center axis, when the disk deforms during rotation, the magnetic head develops a positional displacement, compromising proper reading. Accordingly, to cope with higher rotational speeds described above, the glass substrate is required to have high rigidity (Young's modulus) so as to prevent substantial deformation during high-speed rotation.
Furthermore, the use of a glass substrate with a high coefficient of thermal expansion permits an increase in the reliability of recording and reproduction with magnetic recording media for the following reasons.
HDDs (hard disk drives), in which magnetic recording media are loaded, are configured such that the spindle of a spindle motor presses against the center portion, causing the magnetic recording medium itself to rotate. Thus, when there is a substantial difference in the coefficient of thermal expansion of the substrate of the magnetic recording medium and spindle material constituting the spindle portion, a discrepancy ends up developing between the thermal expansion and contraction of the spindle and those of the substrate of the magnetic recording medium in response to change in the surrounding temperature during use. As a result, the phenomenon in which the magnetic recording medium ends up changing shape occurs. When such phenomenon occurs, information that has been written cannot be read by a head, compromising recording and reproduction reliability. Accordingly, enhancing the reliability of magnetic recording media requires that glass substrates have high coefficient of thermal expansion similar to that of the spindle material (such as stainless steel).
As set forth above, it is required for a glass substrate to have the characteristics of high heat resistance, high rigidity, and a high coefficient of thermal expansion to provide a magnetic recording medium capable of handling higher recording densities. However, based on investigation by the present inventors, conventional glass substrates, including the glass substrates described in Documents 1 to 10 above, do not simultaneously possess these characteristics. This is because there is a trade-off between these characteristics, making it difficult to achieve a glass substrate satisfying all.
A further example of a desirable characteristic of the glass substrate for a magnetic recording medium is a high chemical strengthening property (the ability to readily form an ion-exchange layer when subjected to a chemical strengthening treatment). This is because an ion-exchange layer (compressive stress layer) is often formed on the surface of the substrate by a chemical strengthening treatment to increase the resistance to fracture required for increased reliability of the glass substrate for a magnetic recording medium. The chemical strengthening treatment also has the effect of reducing the amount of alkali eluting from the surface of the glass substrate.